1. Field of the Invention
The present invention relates to cross-linked organic polymers, including those having aromatic groups within the polymer chain, and cross-linking compositions and methods for making such polymers. More particularly, it relates to methods for controlling the cross-linking reaction rate of the cross-linking compounds in such compositions to form high glass transition temperature organic polymers, and to methods for enhancing processability of such cross-linked organic polymers and polymer compositions in forming molded parts which may be used, for example, in down-hole tool applications.
2. Description of Related Art
High glass transition temperature polymers have been useful for a number of high temperature applications. Modification of such high glass transition organic polymers generally improves high temperature performance, strength and chemical resistance for use as parts and articles of manufacture necessary in extreme temperature environments as compared to unmodified organic polymers.
Cross-linking has been widely recognized as one way to modify high temperature polymeric materials. Several inventions have been aimed at improving the high temperature performance of organic polymers, such as those having aromatic groups in the backbone, by using cross-linking within the polymers by cross-linking to itself, grafting cross-linking compounds to the polymer, or incorporating cross-linking compounds into the polymer such as by blending.
U.S. Pat. No. 5,874,516, which is assigned to the Applicant of the present application and is incorporated herein by reference in relevant part, shows poly(arylene ether) polymers that are thermally stable, have low dielectric constants, low moisture absorption and low moisture outgassing. The polymers further have a structure that may cross-link to itself or can be cross-linked using a cross-linking agent.
U.S. Pat. No. 6,060,170, which is also assigned to the Applicant of the present application and is incorporated herein by reference in relevant part, describes the use of poly(arylene ether) polymer compositions having aromatic groups grafted on the polymer backbone, wherein the grafts allow for cross-linking of the polymers in a temperature range of from about 200° C. to about 450° C. This patent discloses dissolving the polymer in an appropriate solvent for grafting the cross-linking group. Such required process steps can sometimes make grafting difficult or not practical in certain types of polymers or in certain polymeric structures, including, e.g., polyetherether ketone (PEEK).
International Patent Application Publication No. WO 2010/019488 A1, which is also assigned to the Applicant of the present application and is incorporated herein by reference in relevant part, shows per(phenylethynyl) arene polymers that are grafted to a second polymer to provide a cross-linked polymeric network.
Previous attempts have also been made to control where crosslinks form along the backbone of high glass transition polymers to garner the desired mechanical properties and high temperature performance. U.S. Pat. No. 5,658,994, of Applicant, incorporated herein by reference in relevant part, demonstrates the use of a poly(arylene ether) in low dielectric interlayers which may be cross-linked, for example, by cross-linking the polymer to itself, through exposure to temperatures of greater than about 350° C. or alternatively by using a cross-linking agent. In this patent and as mentioned in U.S. Pat. No. 5,874,516, cross-linking occurs at the ends of the polymer backbone using known end capping agents, such as phenylethynyl, benzocyclobutene, ethynyl, and nitrite. The degree of cross-linking can be limited with the results of a lower glass transition temperature, reduced chemical resistance and lesser tensile strength.
International Patent Application Publication No. WO 20130/74120 A1 of the Applicant of the present application, also incorporated herein by reference in relevant part, discloses a cross-linking compound as used in the invention described herein, which is blended with an uncrosslinked polymer to achieve a crosslinked organic polymer with a higher glass transition temperature for use in extreme conditions such as in down-hole tool applications.
While cross-linking agents may be effective, there can be difficulty in controlling the rate and extent of cross-linking. Cross-linked organic polymers having aromatic groups in the backbone such as cross-linked polyarylene ether polymers, including cross-linked polyetherether ketone (PEEK), even when made using agents to control cross-linking as described herein are amorphous polymers that function well at high temperature (having a Tg above about 270° C.). The crosslinking provides enhanced chemical resistance to add to the high temperature properties of the base polymers. Cross-linking can be done using techniques as noted in the patents and patent application publications identified above and as described herein using Applicant's techniques. In molding, the controlled cross-linked polymers perform well at about 250° C. (or somewhat below the Tg of the materials). However, as molding temperatures rise, the reaction can accelerate such that full cure may be achieved in less than one minute. Cycle times for injection molded articles, such as tubes, rods or electrical connectors, however, are generally three to five minutes or longer. A full cure in less than a minute can impede the usefulness of conventional molding techniques, such as injection molding or extrusion, in forming molded parts.
Prior art attempts to retard or inhibit and moderate cross-linking reactions using compounds and their reactions are known. See, Vanderbilt Rubber Handbook, 13th ed., 1990, p. 281. However, there is still a need in the art to control and inhibit such reactions, and to improve the ability to process such polymers more easily using traditional molding techniques.